X-ray cine techniques are employed to determine the instantaneous tension and length of the extensors of the ankle in quadrupeds during running. Steel ball bearings introduced at two points in the tendon of Achilles and at the proximal and distal extremes of the muscle allow the instantaneous length of the muscle and tendon to be known at more than 40 intervals of time during the stride cycle. The tendon is calibrated as a force transducer after the running experiments by stretching it in the x-ray beam as the force is simultaneously measured. In both the running and calibration experiments, a length standard (gnomen) is photographed in th same plane as the muscle and tendon markers. The study is designed to test the assumptions and predictions of a mathematical model of the physical scaling of quadrupedal running. These are the first direct measurements of muscle force ever made in intact animals during running.